This present invention relates generally to video display techniques. More specifically, the present invention relates to pulse width modulation methods used with spatial light modulators. Merely by way of example, the invention has been applied to a pulse width modulation method using an expanded bit plane. The methods and techniques can be applied to other applications as well such as liquid crystal displays and the like.
Reflective spatial light modulators (SLMs) are devices that modulate light in a spatial pattern to reflect an image corresponding to an electrical or optical signal. The incident light may be modulated in phase, intensity, polarization, or direction of deflection. A reflective SLM typically includes a two-dimensional array of addressable picture elements (pixels) capable of receiving and reflecting incident light. Source pixel data is first processed by an associated control circuit, then loaded into the pixel array one frame at a time.
In some SLM displays, the color depth or gray scale brightness produced by a given pixel is controlled using various forms of frame modulation methods. On such method of simulating color depth is pulse width modulation (PWM). One bit-per-pixel (bpp) display devices utilize either an “off” state or an “on” state. Thus, in some PWM systems, the length of time during which an individual pixel is either in the off or the on state is varied to produce gray scale images.
For example in one PWM system, a frame rate and matching frame period is determined based on the rate at which images will be displayed. The intensity resolution is determined for each pixel, with black being zero time slices and the smallest, or “least significant bit” (LSB) equaling one time slice. Then, each pixel's intensity is quantized to determine its appropriate on-time during the frame period. For each pixel with a quantized intensity value greater than zero, its on-time during the frame period equals the number of time slices that correspond to the desired pixel intensity.
FIG. 1A is a simplified field-pulse diagram illustrating a conventional display frame for a six-bit PWM technique with a total of 63 LSB fields. The display frame 105 with frame time 100 includes a total of 63 LSB fields 115. FIG. 1B is a simplified field-pulse diagram illustrating a conventional display frame with bits of various sizes. As illustrated in FIG. 1B, the display frame 120 includes bits of various sizes as marked with indicators ranging from 0-5. The shortest bit, referred to as the LSB 125 and marked with a 0, determines the size of the fields by which the various bits 1-5 are measured. The LSB 125 is shown as one LSB field long, as measured against FIG. 1A. The longest bit is referred to as the most significant bit (MSB) 130 and is marked with a 5. The MSB 130 is shown as 32 LSB fields long, as measured against FIG. 1A. The remainder of the bits 135-150 are in between these lengths, specifically, bit 1 (135) is two LSB fields long, bit 2 (140) is four LSB fields long, bit 3 (145) is eight LSB fields long, and bit 4 (150) is sixteen LSB fields long.
In order to address elements of the SLM, the PWM data is arranged in the form of bit planes that match the bit weights of the quantized intensity value. In the simplest instance, the bit planes each are loaded separately during a frame, with the pixels addressed according to their respective bit plane values. For example, the bit plane associated with the LSB of a pixel takes up one time slice in the frame. In contrast, the most significant bit (MSB) may take up several slices in the frame.
The human eye integrates the on and off segments or pulses of light produced by the SLM in a given frame, resulting in a perception of a gray scale brightness value for a given pixel. In general, the greater the number of shades of gray, the better gray scale, or eventually color, resolution is available to a viewer. However, increasing the gray scale resolution generally entails increasing the data rate required to load the data in bit planes. For example, if the number of gray scale resolution values is increased from 7-bit resolution (27=128 shades of gray) to 8-bit resolution (28=256 shades of gray), the data rate may be increased by a factor of two.
In some applications, an intermediate resolution which is greater than a present resolution, but less than a doubled resolution, may be acceptable for a given application. However, conventional methods of PWM as illustrated in FIGS. 1A and 1B do not provide for such intermediate resolutions. Thus, there is a need in the art for improved methods of performing PWM for display applications.